The present invention relates generally to tamper-evident closures and, more particularly, to tamper-evident closures which make it more difficult to conceal the fact that a closure has been opened at extreme temperatures.
Tamper-evident closures have been around for many years and are available in many forms. One such closure is a tamper-evident tape or label which is used to seal conventional boxes, bags and other containers. Another such closure is that found on specially designed security pouches or envelopes, such as those used in banking and other industries. Tamper-evident closures are intended to protect the contents of a package by indicating whether an attempt has been made to open the package and access its contents.
One well-known form of security package is a plastic bag, pouch or envelope (referred to herein as a bag) commonly utilized to transport money, checks, bonds, stocks, food stamps, medical specimens, jewelry and other valuable articles. These bags are typically formed from a plastic film folded in the middle and sealed along opposed side edges to form an enclosure having an open end for inserting articles into the bag. To close the bag, a peel-back strip covering one surface of a pressure-sensitive adhesive is removed, and the exposed adhesive is then pressed against the opposite side of the bag. If an attempt is made to open a bag of this type along the adhesive seal, the pressure-sensitive adhesive or other parts of the bag will distort, providing an indication of the attempted opening. Variants of this type of security bag may include layers of ink, release materials and other materials intended to enhance the ability of the bag to quickly and accurately reveal attempts to open the adhesive seal.
One problem that has arisen with respect to the adhesive materials used to seal these security bags is that the adhesive seals may be secretly opened by subjecting the seals to extreme temperatures. For example, the temperature of the sealed region can be lowered, such as by spraying Freon or another refrigerant onto the sealed region or applying dry ice to the sealed region. Lowering the temperature of the adhesive below its transition temperature causes the adhesive to become brittle and lose its adhesive qualities. The bag can then be opened and its contents accessed or removed. When the adhesive warms back to room temperature, it regains its adhesive properties, and the bag may be resealed without any evidence of tampering. Alternatively, the sealed region may be warmed, such as with a hand-held hair dryer, to soften the adhesive and lessen its adhesive properties. The bag may then be opened and the contents of the bag may be removed. While warm, the adhesive seal may be reclosed. The seal will regain its strength as the adhesive cools and regains its adhesive properties.
In one widely used security bag of this type, the tamper-evident closure includes a transparent or partially transparent substrate, a release material printed in a predetermined pattern on the substrate, and a solid ink printed in a uniform coating over the substrate and release material. A pressure-sensitive adhesive seals the bag in a closed condition. Any attempt to open the bag after it has been sealed will cause the portions of the ink layer overlying the release material to pull away from the substrate, revealing the predetermined pattern in which the release material has been deposited.
When thieves subject the foregoing security bag to refrigerant tampering, they are still confronted with the pattern revealed in the ink layer when the bag is opened. In order to hide this evidence of tampering, the thieves have developed a procedure in which they use a quick-drying ink marker of a color similar to the printed ink to fill in the pattern in the ink layer. Upon resealing of the bag, it is difficult to see that the pattern has been filled in, and the bag appears as if it has not been tampered with.
Although attempts have been made to address the problem of refrigerant tampering, these attempts have not been entirely successful. One reason for this lack of success is that the tamper-evident closures developed to address this problem often have complex structures which are both difficult and costly to manufacture. Another reason for this lack of success is that the evidence of tampering provided by these closures oftentimes is difficult to see without close inspection. Therefore, in environments such as the counting rooms of banks where these security bags are processed in large volumes, evidence that a bag has been tampered with is often overlooked.
There therefore exists a need for a tamper-evident security bag which may be manufactured easily and at low cost, and which provides quick and reliable evidence of tampering, even where a refrigerant or heat source has been used in the tampering process.